Laboratory Performance Characteristicsof Sulfur-Modified Warm-Mix Asphalt

Samuel B. Cooper III1; Louay N. Mohammad, Ph.D., M.ASCE2; and Mostafa A. Elseifi, Ph.D., M.ASCE3

Abstract: The objective of this study was to compare the laboratory mechanistic properties of sulfur-modified warm-mix asphalt (WMA)with conventional asphalt mixtures. Three mixtures, two hot-mix asphalt (HMA) and one WMA, were prepared. Mixture One used anunmodified asphalt binder classified as PG 64-22, Mixture Two used a styrene-butadiene-styrene elastomeric modified binder classifiedas PG 70-22, and Mixture Three was a WMA that incorporated a sulfur-based mix additive and a PG 64-22 binder. A suite of testswas performed to evaluate the rutting performance, moisture resistance, fatigue endurance, fracture resistance, and thermal cracking resis-tance of the three mixtures. Results of the experimental program showed that the rutting performance of sulfur-modified WMA was com-parable or superior to conventional mixes prepared with polymer-modified and unmodified asphalt binders. Results of the modified Lottmantest showed that the moisture resistance of the sulfur-modified mixture was comparable to conventional mixes. Results of the fracture testsshowed that sulfur-modified WMA is more susceptible to cracking than conventional mixes, given its stiff characteristics. However, giventhese stiff properties, the higher modulus of sulfur-modified mixtures will reduce the magnitude of strain induced in the pavement. Thermalstress restrained specimen test results showed that the sulfur-modified WMA had greater fracture stress than the polymer-modified mixture.However, there was no statistical significance between the average fracture temperatures for the mixes tested. DOI: 10.1061/(ASCE)MT.1943-5533.0000303. © 2011 American Society of Civil Engineers.

CE Database subject headings: Sulfur; Mixing; Asphalts; Laboratory tests; Thermal factors.

Author keywords: Sulfur; Warm-mix asphalt; Superpave; Binder; Thiopave; Semicircular bend.

Introduction

During the 1970s and 1980s, attempts were made to use sulfur as abinder extender to reduce the amount of asphalt binder required inmixtures and to improve the mix mechanical characteristics (Timmet al. 2009). The interest in using sulfur in hot-mix asphalt (HMA)was driven by the abundance of this natural resource and the desireto offset the cost of pollution control associated with the stockpilingof this additive (Lee 1975). However, the concept of using sulfur inHMA was abandoned in the 1980s after environmental and safetyproblems were encountered during installation and doubts aboutthe cost viability of the modification were expressed (Deme andKennedy 2004). Segregation of the additive from the binder wasalso reported because of the large difference in density betweensulfur and asphalt binder (Strickland et al. 2008).

In spite of installation difficulties, sulfur modification wasreported to be effective for enhancing the mechanical performanceand stiffness characteristics of the mixture over conventional

mixtures (Kennedy et al. 1977). With the recent increase in theprice of liquid asphalt, a petroleum-based product, the use of sulfuras a binder extender appears economically attractive. The conceptof sulfur extended asphalt (SEA) reappeared with the developmentof a new generation of a solid dust-free sulfur product, known asThiopave. Many of the earlier safety problems appear to have beensolved, as long as the mixture is produced at a target mixing tem-perature of 135� 5°C (Taylor et al. 2010). In addition, the newadditive does not need to be preblended in the plant with thehot binder, because it is added during mixing of the aggregates afterthe asphalt binder is added. Because sulfur-modified asphalt mix-ture needs to be produced at a mixing temperature that is lower thanthe required mixing temperature for conventional HMA, the newgeneration of SEA should be used with an asphalt mixture witha lower mixing temperature such as warm-mix asphalt (WMA).Because WMA is designed to reduce mixing temperature duringproduction to 16 to 55°C lower than typical HMA, the use of sulfurin the production of WMA may offer the potential to reduce energyand asphalt consumption in the preparation of asphalt mixtures.

To evaluate the effectiveness of the new generation of sulfur ad-ditives, the objective of this study was to compare the mechanicalproperties of sulfur-modified WMAwith conventional asphalt mix-tures. A commonly used wearing course mixture was prepared bymixing aggregate blends with two virgin binders, a straight asphaltbinder classified as PG 64-22 and a polymer-modified binderclassified as PG 70-22. Laboratory testing evaluated the rutting per-formance, moisture resistance, fatigue performance, fracture resis-tance, and thermal cracking resistance of the produced mixtures byusing the Hamburg loaded-wheel tester (LWT), the flow number(Fn) test, the repeated shear at constant height (RSCH) test, themodified Lottman test, the beam fatigue test, the semicircular bend(SCB) test, the dissipated creep strain energy (DCSE) test, and thethermal stress restrained specimen test (TSRST).

1Graduate Research Assistant, Dept. of Civil and EnvironmentalEngineering, Louisiana State Univ.

2Irma Louise Rush Stewart Distinguished Professor, Dept. of Civiland Environmental Engineering, Director, Engineering Materials Charac-terization Research Facility, Louisiana Transportation Research Center,Louisiana State Univ., 4101 Gourrier Ave., Baton Rouge, LA 70808(corresponding author). E-mail: louaym@lsu.edu.

3Assistant Professor, Dept. of Civil and Environmental Engineering,Louisiana State Univ.

Note. This manuscript was submitted on November 30, 2010; approvedon March 2, 2011; published online on August 15, 2011. Discussion periodopen until February 1, 2012; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 23, No. 9, September 1, 2011. ©ASCE, ISSN 0899-1561/2011/9-1338–1345/$25.00.

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Background

The original experiences with sulfur-extended asphalt was reportedin literature (Lee 1975; Kennedy et al. 1977; Mahoney et al. 1982).Sulfur was typically added to the asphalt binder at a mixing temper-ature of 150°C at a ratio ranging from 1=5 to greater than 1. At alow modification rate, sulfur influenced the rheological propertiesof the binder, which resulted in an increase in binder stiffness and areduction in viscosity and ductility. At high sulfur content, the ad-ditive acted as filler and improved the workability of the mixtureand its mechanical strength. At a modification rate approaching50%, SEA improved the mixture engineering properties over con-ventional asphalt mixtures (Kennedy et al. 1977).

Recent investigations of the new modified sulfur mix additivetechnology were reported in literature (Strickland et al. 2008; Timmet al. 2009). The sulfur additive, usually added at a ratio rangingfrom 30 to 40% of the binder weight, consists of pretreated solidpellets that melt at a temperature above 120°C. The pellets are pre-treated to reduce emissions of harmful pollutants such as hydrogensulfide gas during production and to lower mixing and compactiontemperature required for the modified mixture. During mixing, partof the sulfur dissolves into the binder at a high temperature andreduces its viscosity. The remaining part precipitates out as the mix-ture cools and crystallizes as part of the asphalt matrix surroundingthe coarse aggregate. These sulfur crystalline particles stiffen themixture and act as a strengthening agent at high temperature,resulting in improved rutting resistance. Sulfur modification alsoacts as a binder extender, which results in a decrease in the required

binder content in the mixture by approximately 20 to 25% by vol-ume (Kentucky Production Evaluation List 2010). Given the differ-ence in density or specific gravity between the binder and sulfur, itis recommended to maintain the volume fraction of the total binderphase in the modified mixture based on the following relationship(Strickland et al. 2008):

Sulfur þ Binder% ¼ 100AR100R� PsðR� GbinderÞ

ð1Þ

where Sulfur + Binder % = binder and sulfur content in the mixture;A = percentage of binder by weight in conventional mixture (%);R = sulfur to binder specific gravity ratio (approximately 2.0); Ps =weight percentage of sulfur in the modified blend; and Gbinder =specific gravity of the unmodified binder.

Strickland and coworkers (2008) evaluated the performance ofsulfur-modified mixtures in the laboratory. Rutting performance ofthe prepared mixtures was evaluated using the asphalt pavementanalyzer (APA) test at 58°C, and the mixture stiffness moduluswas measured at a temperature ranging from �10 to 30°C. In ad-dition, the low temperature performance was evaluated by using theTSRST. Results of this analysis indicated that the rutting resistanceand stiffness modulus of the mixture are improved. In addition, themodified sulfur additive enhanced the elongation properties of themix at low temperatures. A comprehensive experimental programalso evaluated the moisture resistance and dynamic modulus of sul-fur-modified asphalt mixtures (Timm et al. 2009). Results showedthat sulfur-modified asphalt mixture had a lower tensile-strength

Table 1. Job Mix Formula of the Asphalt Mixtures

Mixture designation WC70CO WC64CO WC64SU

Mix type 19.0 mm (3=4 in) Superpave

Aggregate #67 LS 36% 36% 36%

#78 LS 24% 24% 24%

#11 LS 34% 34% 34%

CS 6% 6% 6%

Binder type PG 70-22M PG 64-22 PG 64-22

Binder Content (%) 4.0 4.0 3.0a

% Gmm at NIni 87.0 87.0 87.0

% Gmm at NMax 97.6 97.6 97.6

Design air void (%) 3.7 3.7 3.7

VMA (%) 13 13 13

VFA (%) 68 68 68

Metric sieve Composite gradation blend

37.5 mm (1 1/2 in) 100 100 100

25.0 mm (1 in) 100 100 100

19.0 mm (3=4 in) 96 96 96

12.5 mm (1=2 in) 75 75 75

9.5 mm (3=8 in) 59 59 59

4.75 mm (No. 4) 43 43 43

2.36 mm (No. 8) 31 31 31

1.18 mm (No. 16) 20 20 20

0.600 mm (No. 30) 11 11 11

0.300 mm (No. 50) 8 8 8

0.150 mm (No. 100) 6 6 6

0.075 mm (No. 200) 4.5 4.5 4.5

Note: Limestone (LS); coarse sand (CS).a60=40 KB: WMA with 60% PG 64� 22þ 40% Thiopave additive.

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ratio (TSR) after curing but greater dynamic moduli for all combi-nations of test temperatures and frequencies.

Cooper et al. (2011) evaluated the effects of sulfur-modifiedWMA on the predicted performance from the Mechanistic-Empirical Pavement Design Guide (MEPDG) and assessed the lifecycle costs of pavement structures constructed with this sustainablealternative. To achieve this objective, three typical pavement struc-tures were analyzed at three traffic levels (low, medium, and high).Based on the results of the analysis, the use of sulfur-modifiedWMA improved the predicted rutting and fatigue performancesand the overall pavement service lives over conventional mixturesat all traffic levels. Results also indicated that sulfur modificationhas the potential to reduce production and life cycle costs whencompared to a conventional asphalt mixture prepared with the samebinder grade (Cooper et al. 2011).

Experimental Program

Awearing course mixture with a nominal maximum aggregate size(NMAS) of 19 mm was prepared by mixing aggregate blends withtwo binders: an unmodified binder classified as PG 64-22 (hereafterreferred to as WC64CO) and a styrene-butadiene-styrene (SBS)elastomeric modified binder classified as PG 70-22 (hereafter re-ferred to as WC70CO). This mixture is a Superpave Level 2 designthat was performed according to AASHTO PP 28 (2000c) andSection 502 of the Louisiana Standard Specifications for Roadsand Bridges (2006). Specifically, the optimum asphalt cement con-tent was determined to be 4% based on the following volumetriccriteria [void in the total mixture (VTM) = 2:5� 4:5%; voids in themineral aggregate (VMA) ≥ 12%; voids filled with asphalt (VFA)= 68%� 78%] and densification requirements (%Gmm atN initial ≤ 89, %Gmm at Nfinal ≤ 98). The job mix formula for thethree mixtures utilized in this study is summarized in Table 1.The same crushed siliceous limestone aggregate structure was usedin all the mixtures evaluated in this study. Mixture Three, referredto in this paper as WC64SU, was a WMA prepared with sulfur-based additives and PG 64-22 binder. Forty percent of the totalbinder weight was replaced with the sulfur additive. Hence, the de-sign PG 64-22 binder content was reduced from 4.0 to 3.0% bytotal weight of the mix, according to Eq. (1). This represents a25% reduction in asphalt binder content compared with MixturesWC64CO and WC70CO. For compatibility reasons, the sulfur ad-ditive is only recommended for use with unmodified asphalt binder.

The mixing temperature for Mixtures WC64CO and WC70COwas 163°C and the mixing temperature for WC64SU was 140°C.The mixing process for the sulfur-modified mixture is illustratedin Fig. 1. The binder preparation consists of adding a compactionadditive (CA 100) at 1.0% and an antistripping agent (Akzo NobelKB 2550) at 1.0% of the heated binder weight. The compactionadditive allows the mixture to be prepared at a lower temperaturethan with conventional HMA. The sulfur pellets used in theWC64SU mixture were heated to 60°C before addition to theaggregate-binder blend. The heated binder along with the compac-tion agent and antistrip additive were then mixed with the hotaggregates (140°C), followed by adding the preheated sulfur pelletsand mixing thoroughly to ensure that all pellets were melted.

The research team conducted a preliminary factorial to deter-mine the optimum proportions of sulfur additives. The LWT andSCB tests were conducted as part of the screening factorial. Awear-ing course and a base course mixture were evaluated using a sulfurcontent ranging from 30 to 40% by weight. The optimum percent-age of sulfur additive was determined to be 40%.

Performance Testing

Laboratory performance testing included evaluation of the ruttingperformance, moisture resistance, fatigue performance, fracture re-sistance, and thermal cracking resistance of the prepared asphaltmixtures by using the Hamburg LWT, Fn test, RSCH test, modifiedLottman test, beam fatigue test, SCB test, DCSE test, and TSRST.Table 2 presents the test factorial conducted for the three mixturesevaluated in this study and the number of specimens tested. Trip-licate specimens were considered for each test, except for the LWT,where two specimens were tested. All specimens were compactedto an air void level of 7� 1%. Results of the tests presented inTable 1 had a coefficient of variation that was less than 15%.A brief description is presented of each of the test methods con-sidered in the experimental program.

Semicircular Bending Test

Cracking potential was assessed by using the SCB test (Wu et al.2005). This test characterizes the fracture resistance of HMAmixtures based on fracture mechanics principals, the critical strainenergy release rate, also called the critical value of J-integral, or Jc.To determine the critical value of J-integral (Jc), semicircular spec-imens with at least two different notch depths should be tested foreach mixture. In this study, three notch depths of 25.4, 31.8, and38 mm were selected based on an a=rd ratio (the notch depth tothe radius of the specimen) between 0.5 and 0.75. Test temperaturewas selected to be 25°C. The semicircular specimen was loadedmonotonically until fracture failure under a constant cross-headdeformation rate of 0:5 mm=min. in a three-point bending loadconfiguration. The load and deformation are continuously recordedand the critical value of J-integral (Jc) is determined by using thefollowing equation (Wu et al. 2005):

Jc ¼�U1

b1� U2

b2

�1

a2 � a1ð2Þ

where b = sample thickness; a = notch depth; and U = strain energyto failure.

Fig. 1. Illustration of the laboratory preparation of the sulfur-modifiedasphalt mixture

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Loaded-Wheel Tracking Test

Rutting performance of the mixes was assessed by using aHamburg-type LWT, manufactured by PMW, Inc. of Salina,Kansas. This test was conducted according to AASHTO T 324(2008b). The test applies a repetitive load on slab specimens com-pacted to 7� 1% air voids that have a length of 320 mm, a width of260 mm, and a thickness of 80 mm. This test is considered a torturetest that produces damage by rolling a 703-N steel wheel across thesurface of a slab that is submerged in 50°C water for 20,000 passesat 56 passes a minute. A maximum allowable rut depth of 6 mmafter 20,000 passes at 50°C is required. Therefore, the rut depth at20,000 cycles was measured and used in the analysis.

Flow Number Test

The Fn test was used to assess the permanent deformation charac-teristics of paving materials by applying a repeated dynamic loadfor several thousand repetitions on a cylindrical asphalt sample.The Fn is defined as the starting point, or cycle number, at whichtertiary flow occurs on a cumulative permanent strain curveobtained during the test. This test uses a loading cycle of 1.0 s induration and consists of applying 0.1-second haversine load fol-lowed by 0.9-second rest period (Bonaquist et al. 2003). Permanentaxial strains are recorded throughout the test. The test was con-ducted at an effective temperature (Teff ) of 54°C and a stress levelof 207 kPa. This test is applicable to laboratory prepared specimensthat are 100 mm in diameter and 150 mm in height for mixtureswith nominal maximum size aggregate less than or equal to37.5 mm.

Repeated Shear at Constant Height Test

This test was conducted according to AASHTO TP7 (2000a)Procedure F, which is a stress-controlled test that applies haversineshear stress pulses to a cylindrical specimen. The shear stress am-plitude is applied with a maximum shear stress of 68 kPa for a load-ing time of 0.1 s and a rest period of 0.6 s. A varying axial load isapplied automatically during each cycle to maintain the specimen atconstant thickness or height. Repetitive loading is applied for a totalof 5,000 repetitions or until 5% permanent shear strain is reached.The primary response variable from this test is the cumulative per-manent shear strain at the end of the test.

Modified Lottman Test

This test was conducted according to AASHTO T 283 (2003),which evaluates the effect of saturation and accelerated water con-ditioning on compacted HMA specimens. This method quantifiesHMA mixture sensitivity to moisture damage, which is necessaryto assure durable and long lasting mixtures. Numerical values ofretained indirect-tensile properties are obtained by comparingconditioned samples, specimens subjected to vacuum saturationand freeze-thaw cycles, to unconditioned specimens. Uncondi-tioned samples are not saturated. For each mix used in thestudy, six 150 × 95-mm diameter specimens were prepared. The

unconditioned specimen subset was stored at room temperaturefor 24� 3 hours. Afterward, the unconditioned specimens werewrapped or placed in a heavy duty leakproof plastic bag and thenplaced in a 25� 0:5°C water bath for 2 h� 10 min. The uncon-ditioned specimens were then tested to determine the indirect ten-sile strength (ITS) for each sample. The conditioned specimenswere placed in a freezer at �17:7°C for 16 to 18 h. After the freez-ing cycle, the conditioned samples were placed in a 60°C water bathfor 24 h. Upon completion of the freeze/thaw cycle, the indirecttensile strength for the conditioned specimens was determined at25� 0:5°C. The average ITS was determined for both conditionedand unconditioned specimens by summing the test values and thendetermining the average value. The TSR is defined as the ratio ofconditioned to unconditioned ITS.

Dissipated Creep Strain Energy Test

The DCSE threshold represents the energy that the mixture can tol-erate before it fractures (Roque et al. 2004). DCSE evaluation of anHMA mixture involves two individual laboratory tests to be per-formed on the same specimen. The indirect tensile resilient modu-lus (MR) test and the ITS test were conducted at 10°C on the samespecimen to calculate the dissipated creep strain energy. Triplicatespecimens of 150 mm in diameter and 50 mm in thickness wereused. The test specimens were conditioned at 10°C for 4 h beforea 200-cycle haversine load with 0.1-s loading period and 0.4-s restperiod in each loading cycle was applied along the diametricalplane of the specimen. A conditioning loading sequence was ap-plied before the start of the actual test to obtain uniform measure-ments in load and deformation. A four-cycle haversine compressiveload was then applied and load and deformation data were recordedcontinuously. The magnitude of the applied load was selected suchthat it results in a deformation as close as possible to 100 micro-strains. After one test was completed, the specimen was rotated 90°and tested again. The resilient modulus was calculated from theaverage value of the two test results. Once theMR test was finished,the ITS test was then performed on the same specimen.

The DSCE is defined as the fracture energy (FE) minus theelastic energy (EE) (Roque et al. 2004). The fracture energy isdefined as the area under the stress-strain curve up to the pointwhere the specimen begins to fracture. Mull et al. (2002) foundthat the DCSE is a good predictor of the cracking performanceof HMA. A summary of the DCSE calculation is described asfollows:

Mr ¼St

εf � ε0ð3Þ

ε0 ¼ðMR × εf � StÞ

MRð4Þ

EE ¼ 12× St × ðεf � ε0Þ ð5Þ

Table 2. Experimental Test Factorial

Mix type Mixture variables Modified Lottman LWT Fn RSCH DCSE TSRST Fatigue SCB

19 mm NMAS SP Level 2 Mixture ID Sulfur content Unconditioned Conditioned Aged Aged

WC64CO 0 3 3 2 3 N/A 3 N/A N/A 3

WC70CO 0 3 3 2 3 3 3 3 3 3

WC64SU 40% 3 3 2 3 2 3 3 3 3

Total 9 9 6 9 5 9 6 6 9

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DCSE ¼ FE–EE ð6Þwhere St = peak tensile strength; ε0 = initial strain; εf = failurestrain; EE = elastic energy; and FE = fracture energy. Roque et al.(2004) reported a DCSE value of 0:75 KJ=m3 as the limiting cri-terion for acceptable fracture resistance. HMAmixtures with DCSEvalues greater than 0:75 KJ=m3 are not susceptible to cracking.

Thermal Stress Restraining Specimen Test

Low-temperature cracking performance was assessed using theTSRST, which was used to study changes in the binder’s glass tran-sition temperature owing to the use of sulfur and how it may affectthe mix performance in terms of low temperature cracking. This testwas conducted according to AASHTO TP 10 (2000b). Triplicatespecimens 50 mm thick × 50 mmwide × 254 mm long were used.The specimen is glued at the ends to two aluminum platens. Thetest device then cools the beam specimen while restraining it fromcontracting. As the temperature drops, thermal stresses build upuntil the specimen fractures. If the glass transition temperaturechanges significantly owing to the addition of sulfur, one wouldobserve an increase in the temperature at which fracture occurs.

Beam Fatigue Test

This test was conducted according to the AASHTO T 321(2008a) test method at 20°C. The test was conducted in a strain-controlled mode in which a beam 318 mm long × 63:5 mmwide ×50:8 mmhigh is subjected to third-point bending at a frequency of10 Hz. The strain level was selected such that failure would notoccur before 10,000 cycles. The center deflection of the beamwas continuously measured and used in the computation of thebeam stiffness. Failure was defined as the load cycle at whichthe specimen exhibits a 50% reduction in stiffness. The numberof cycles to failure (Nf ) was computed and used in the analysis.The reported test results were the average of three test samples.

Results and Analysis

Rutting Performance

Fig. 2 compares rutting performance of the three mixes evaluatedin this study. This test predicts an acceptable rut performance for amixture that achieves a maximum rut depth of 6.0 mm after 20,000passes. As shown in Fig. 2, Mixture WC64CO had the largestrut depth at 20,000 cycles, followed by WC64SU and WC70CO.Mixtures WC70CO and WC64SU exhibited a rut depth at20,000 cycles that is less than 6.0 mm, shown in Fig. 2.

Fig. 3 presents the Fn for the three mixtures evaluated in thisstudy. The Fn is defined as the number of cycles at which tertiaryflow occurs on a cumulative permanent strain versus number of

cycles curve. The greater the Fn, the higher the mixture’s resistanceto permanent deformation. As shown in Fig. 3, the sulfur-modifiedWMA mixture (WC64SU) outperformed both conventional mix-tures, including the polymer-modified mix, in its resistance topermanent deformation.

Fig. 4 presents the results of the RSCH test for the WC70COand WC64SU mixtures. The permanent shear strain at 5,000 cyclesis used to evaluate the mixture’s susceptibility to permanent defor-mation (Sousa et al. 1994). In this test, a lower permanent shearstrain value is indicative of a reduced susceptibility to ruttingfailure. As shown in Fig. 4, the sulfur-modified WMA mixture(WC64SU) had lower permanent shear strain at 5,000 cyclesthan the conventional polymer-modified mixture (WC70CO). Insummary, the permanent deformation tests ranked the WC64SU

02468

101214161820

0 5000 10000 15000 20000

Rut

Dep

th (

mm

)

Number of Passes

WC64COWC70COWC64SU

Fig. 2. LWT test results: 50°C

Fig. 3. Flow number test results

0.00

0.01

0.02

0.03

0.04

0.05

0 1000 2000 3000 4000 5000

Per

man

ent S

hear

Str

ain

at 5

,000

C

ycle

s

Number of Cycles

WC64SU WC70CO

Fig. 4. Repeated shear at constant height test results

Fig. 5. Critical strain energy data from the SCB test

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as better or similar to WC70CO in terms of rutting resistance.These findings agree with other investigators, who have reportedthat the use of sulfur modification improves the permanent defor-mation resistance of the mixture at high temperatures (Stricklandet al. 2008; Taylor et al. 2010). The improved rutting performanceis attributable to the stiffening effect of the sulfur crystals, whichacts as a structuring agent for mixtures at high temperatures.

Fracture Performance

Fig. 5 presents a comparison of the critical strain energy (Jc) datafor the three mixtures evaluated in this study. High Jc values aredesirable for fracture-resistant mixtures. The fracture resistancesof conventional asphalt mixtures were greater than those of thesulfur-modified WMA mixture. Because a threshold of a minimumJc of 0:65 kJ=m2 is typically used as a failure criterion for this test(Wu et al. 2005), it appears that the sulfur-modified mixture and theconventional mixture prepared with the PG 64-22 unmodifiedbinder do not meet the cracking criterion set for this test. Giventhat cracking resistance is primarily controlled by the binder in themixture, it is possible that the use of sulfur reduced the ductility andelongation properties of the binder at intermediate temperature.However, a different cracking performance may be observed if adifferent rate of sulfur modification or softer base asphalt is used,as reported in past studies (Strickland et al. 2008). Given its stiffcharacteristics, the sulfur-modified WMA mixture can be a candi-date in a perpetual type of thick pavement structure or where astiffer asphalt mixture is desirable.

Fig. 6 presents the mean DCSE values for the mixtures evalu-ated in this study. Mixtures that exhibit lower DCSE values aremore susceptible to cracking than mixtures with higher values.As shown in this figure, the WC70CO mixture had the highestDCSE values, and is therefore less prone to cracking at the testedtemperature of 10°C. However, all mixtures met the 0:75 KJ=m3

cracking criterion set for this test (Roque et al. 2004).

Fatigue Performance

Fig. 7 illustrates the relationship between the number of cyclesto failure and the strain level applied to the specimen; MixtureWC64CO was not tested in fatigue. As shown in Fig. 7, thepolymer-modified mix (WC70CO) had a flatter slope, which indi-cates that the modified mix will exhibit greater fatigue life at ahigher bending strain. Other investigators have also reported that

the average fatigue life of conventional mixtures in the laboratoryis longer than sulfur-modified mixes at high strain levels (Timmet al. 2009). This may be caused by the stiffening effect of the sulfuradditive owing to the crystallization of the sulfur particles duringmixing, and by the part of the sulfur that dissolves into the binderduring mixing and reduces its ductility. The results from thistest are consistent with those observed from the SCB test.However, given its stiff properties, the higher modulus of sulfurmodified mixtures will reduce the magnitude of strain inducedin the pavement. This may have a positive effect on the mix fatigueperformance in real pavements, which are subjected to stress-controlled vehicular loading instead of strain-controlled loading, aswas simulated in the laboratory (Cooper et al. 2011).

Low Temperature Cracking Performance

Fig. 8 presents the results of the thermal stress restrained specimentest; Mixture WC64CO was not tested. As shown in Fig. 8, MixtureWC70CO had a lower fracture temperature than Mixture WC64SU.However, statistical analysis presented in the next section showedthat this difference was not statistically significant. The differencein fracture temperature may be caused by the stiffening effect ofthe sulfur on the binder, which reduced its ductility and its abilityto dissipate the applied stress at low temperatures. This may alsoindicate that the glass transition temperature of the binder increasedowing to the sulfur modification, which increased the critical tem-perature at which fractures were observed.

Fig. 6. Dissipated creep strain energy test results

Nf = 5x10-10ε-3.9062

R² = 0.82

Nf = 2x10-11ε-4.71

R² = 0.99

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07

Ben

ding

Str

ain

Lev

el (

10-6

)

Number of Cycles to Failure (Nf)

WC64SUWC70CO

Fig. 7. Beam fatigue test results

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Moisture Resistance Performance

Fig. 9 presents the moisture resistance performance of the threemixtures based on the modified Lottman test results. Although itappears from these results that the sulfur-modified WMA hadgreater TSR than the conventional mixes, statistical analysispresented in the next section showed that this difference was notstatistically significant. Considering that an 80% minimum TSRis necessary, the WC64CO mix would fail this requirement.

Summary of the Results

The tests evaluated and presented were selected to capture thelaboratory performance of sulfur-modified WMA compared to con-ventional mixtures. However, because of the experimental nature ofthe research, test results did not consistently rank the evaluatedmixtures. This may be because of the differences in loading con-figurations and test procedures adopted in each test setup. Forinstance, the LWT samples are tested in a confined mode, whereasthe Fn test is conducted in an unconfined mode.

To address this concern, laboratory test results were statisticallyanalyzed and grouped using the ANOVA procedure provided in the

Statistical Analysis System (SAS) program. A multiple comparisonprocedure with a significance level of 5% was performed for themeans. The results of the statistical grouping were reported withthe letters A, B, C, and so forth. The letter A was assigned to thebest performer, followed by the other letters in the appropriateorder. A double (or more) letter designation, such as A/B, indicatesthat the difference in the means is not clear-cut, and that the resultscould fall in either category.

Table 3 summarizes the statistical ranking of the laboratorytest results for the three mixture types considered in this study.In general, WC64SU performed better than WC64CO, exceptfor the results of the SCB test. Furthermore, the high temperatureproperties of the WC64SU mixture were similar to or better thanthe WC70CO mixture. TSRST results showed that the sulfur-modified WMA mixture had greater fracture stress than thepolymer-modified mixture. However, there was no statistical sig-nificance between the average fracture temperatures for the mixestested. With respect to fatigue performance, the average fatiguelife of the polymer-modified mixtures was longer than the sulfur-modified mix.

Summary and Conclusions

This study evaluated the effectiveness of a new generation ofa modified sulfur mix additive in the production of warm-mixasphalt. A Superpave 19-mm wearing course asphaltic concretemixture was designed and evaluated. Three mixtures, two HMAsand one WMA, were prepared. Mixture One included an unmodi-fied asphalt binder classified as PG 64-22, Mixture Two containedan SBS elastomeric modified binder classified as PG 70-22,and Mixture Three was a WMA that incorporated Thiopave(sulfur-based) additives and PG 64-22 binder. A suite of testswas performed to evaluate the rutting performance, moisture re-sistance, fatigue endurance, fracture resistance, and thermal crack-ing resistance of the three mixtures using the Hamburg-typeloaded-wheel tester, the flow number test, the repeated shear atconstant height test, the modified Lottman test, the beam fatiguetest, the semicircular bending test, the dissipated creep strainenergy test, and the thermal stress restrained specimen test. Basedon the results of this analysis, the following conclusions maybe drawn:1. Results of the LWT, RSCH, and Fn tests showed that the

rutting performance of the sulfur-modified WMA mixture wascomparable or superior to the conventional mixtures preparedwith polymer-modified and unmodified asphalt binders.

2. Results of the modified Lottman test showed that the sulfur-modified WMA had comparable moisture resistance to theconventional mixes.

3. Results of the fracture tests showed that sulfur-modified WMAis more susceptible to cracking than conventional mixes, givenits stiff characteristics. However, given its stiff properties, thehigher modulus of sulfur-modified mixtures will reduce themagnitude of strain induced in the pavement.

Table 3. Summary of Test Results

Moisture resistance Rutting Fatigue Fracture Low temperature

Mix ID TSR LWT RSCH Fn Fatigue DSCE Jc TSRST (fracture temperature) TSRST (fracture stress)

WC64CO Aa B N/A C N/A B B N/A N/A

WC70CO A A A B A A A A B

WC64SU A A A A B B C A AaThe letter A was assigned to the best performer, followed by the other letters in appropriate order.

Fig. 8. Thermal stress restrained specimen test results

Fig. 9. Modified Lottman test results

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4. TSRST results showed that the sulfur-modified WMA mixturehad greater fracture stress than the polymer-modified mixture.However, there was no statistical significance between theaverage fracture temperatures for the mixes tested.On the basis of the results presented in this study, it is recom-

mended that WMA mixes with different levels of sulfur modifica-tion should be evaluated. Field evaluation of these mixes shouldalso be conducted before implementation.

Acknowledgments

The support provided by the Louisiana Transportation ResearchCenter (LTRC) and Shell, Inc. is greatly appreciated. The contentsof this paper do not necessarily reflect the official views or policiesof the Louisiana Department of Transportation and Development orthe Louisiana Transportation Research Center.

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